A number of neurological and neuropsychiatric disorders, including fragile x syndrome, autism spectrum disorders, and drug addiction, may result from altered regulation of microRNAs. MicroRNAs (miRs) are small, conserved, noncoding RNAs that act in association with the RNA-induced silencing complex (RISC) to regulate gene expression post-transcriptionally. MicroRNAs are hypothesized to play a critical role in activity-regulated mRNA translation that controls neuronal development and synaptic plasticity. Our recent work has discovered a novel molecular mechanism for neurotransmitter-regulated protein synthesis that involves the release of a microRNA induced silencing complex (miRISC) from its target mRNA. This proposal will investigate whether this is used as a general mechanism for microRNAs to modulate activity mediated mRNA translation in neurons. We will identify new microRNAs and target mRNAs that utilize reversibility of the miRISC complex to regulate activity-mediated translation. A key question to be addressed is how different neurotransmitter signaling pathways may differentially affect microRNA/RISC targeting to mRNAs. Lastly, we will assess a role for phosphorylation of the fragile x mental retardation protein, FMRP, as a mechanism to regulate a subset of microRNAs at synapses. This research is expected to uncover new molecular mechanisms for activity mediated gene expression that allows for selective, dynamic and spatiotemporal control. The proposal will fill a critical gap in our understanding by characterization of a unifying molecular mechanism allowing for activity- regulated and sequence specific mRNA translation. Aim 1 will use candidate analysis and microarrays to test the hypothesis that neurotransmitter receptor signaling alters the interactions of microRNA/RISC complexes from target mRNAs as a novel mechanism for activity-regulated mRNA translation. Aim 2 will test the hypothesis that a subset of microRNAs and their regulated targeting to mRNAs by gp1 mGlu signaling is dysregulated in a mouse model of fragile x syndrome. This research has the potential to identify novel microRNAs regulated by neuronal activity, understand a unifying molecular mechanism for activity-regulated translation and help to elucidate the pathophysiology of a neurological disorder at the molecular level. This research is expected to have a broad impact on understanding how the posttranscriptional regulation of gene expression is dynamically controlled by RISC and miRNAs in response to neuronal activity to promote neuronal function, which may be altered in neurodevelopmental, neuropsychiatric disorders and drug addiction. These studies are envisioned to have important implications for future therapeutic strategies to manipulate activity-regulated protein synthesis in the treatment of neurological disorders and drug addiction.